Their rookie draft class, however, remains a different story. While Jacobs' short-yardage experience is a needed asset, the 49ers' rookies remain untested, raw talents.

None of their top draft picks has played in the 3-1 start. That isn't a condemnation of their potential but rather a testament to the 49ers' perceived depth.

Wide receiver A.J. Jenkins, running back LaMichael James and guard Joe Looney have been inactive every game, aside from Jenkins in a season opener which he never entered against the host Green Bay Packers.

Jenkins has been too busy as a scout-team receiver to display even a hint of dissatisfaction with his role. "I'm great," Jenkins said. "It's my role right now. I come out every day and give the defenses the best looks so that can help them on game days."

In preparation for Sunday's visit by the Buffalo Bills, Jenkins has played the role of Bills receiver Donald Jones. His previous impersonations: Jordy Nelson (Packers), Calvin Johnson (Lions), Michael Jenkins (Vikings) and Santonio Holmes (Jets).

Yes, someday Jenkins may become the outstanding player that coach Jim Harbaugh promised in training camp. For now, Jenkins and his fellow rookies are "redshirts," to borrow a phrase from their recent college days.

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The only rookies to play this season are sixth-round safety Trenton Robinson on special teams in the first couple games, and undrafted tight end Garrett Celek for a couple snaps in Sunday's 34-0 shutout of the New York Jets.

Harbaugh claimed the rookies are "not solely" biding their time by learning NFL schemes.

"I know they want to play, and they're demonstrating that on the field that they want that," Harbaugh said. "Nobody's hiding. Nobody's trying not to be seen out there on the practice field. They're jumping in there and they're working extremely hard."

Jenkins is buried on a receiving depth chart in which decorated veteran Randy Moss is only seeing about 20 snaps per game. James, Oregon's all-time leading rusher, offers a change-of-pace style that Kendall Hunter already is flourishing with behind starter Frank Gore.

"They're practicing very well and are certainly game ready," offensive coordinator Greg Roman said. "It's just a matter of the numbers and where the need is that particular week. Nothing's out of the realm of possibility at any time."

Looney, a fourth-round pick, missed offseason work with a foot injury while Alex Boone seized the starting spot at right guard. The other draft picks: linebacker Darius Fleming (fifth round) is on the physically-unable-to-perform list after tearing an anterior cruciate ligament; center Jason Slowey (sixth round) was cut in training camp; and linebacker Cam Johnson is on the practice squad.

Jones, the Bills' second-leading receiver, was signed in 2010 as an undrafted free agent out of Youngstown State, where the 49ers' practiced last week before visiting the Jets.

The St. Louis Rams' 17-3 win Thursday night over the previously unbeaten Arizona Cardinals gave the NFC West a 12-6 record this season: Cardinals 4-1, 49ers 3-1, Rams 3-2, Seahawks 2-2. Those teams' combined record last season through as many games: 5-13.

Neither linebacker Aldon Smith nor right tackle Anthony Davis drew fines from the league despite their unnecessary-roughness penalties in Sunday's 34-0 win over the Jets. Also avoiding financial persecution was safety Dashon Goldson, who delivered a fumble-forcing hit on tight end Dedrick Epps' knee.